The present invention relates to a platemaking apparatus for a stencil screen used in screen printing and, in particular, to a stencil screen platemaking apparatus of a heat-sensitive platemaking method in which a thermal head contacts a screen with a heat-sensitive layer so that heating perforation is performed.
As a platemaking method for stencil screens, a photoengraving method has widely been used in which a positive film is placed on a silk screen provided with photosensitive emulsion thereon so that exposure and subsequent development are performed. Nevertheless, this method requires a large amount of effort and time. Thus, in recent years, a heat-sensitive platemaking method has been used in which heating perforation is performed by a thermal head.
In this heat-sensitive platemaking method, a screen fabricated by stacking a heat-sensitive layer composed of a polyester film having a thickness of 1 μm or the like onto fabric of silk mesh is stretched on a rectangular screen frame composed of aluminum alloy or the like. Then, a thermal head scans the surface so that heating perforation is performed on the heat-sensitive layer into a desired pattern. The heating perforation is performed through a controller that controls the heating of a resistor of the thermal head on the basis of input data of a platemaking pattern generated on a computer or loaded into the computer. This avoids a large amount of effort, and permits remarkably rapid platemaking. Further, this provides the advantage of easy correction, change, and the like of the platemaking pattern.
Nevertheless, the above-mentioned art work material for platemaking generates a step between the screen frame having a certain thickness and the thin screen. Thus, in a prior art stencil screen platemaking apparatus of a heat-sensitive platemaking method, an elastic plate or the like composed of rubber having a thickness corresponding to the step has been arranged as backing inside the screen frame, so that the thermal head has been moved by ensuring a pushing pressure against the heat-sensitive surface. Or alternatively, a platen roller has been arranged on the inner side of the screen, so that the platen roller and the thermal head pinch the screen and move in synchronization.
Nevertheless, in every prior art stencil screen platemaking apparatus of a heat-sensitive platemaking method, platemaking has been performed through scanning by the thermal head in a state that the screen-stretched screen frame is arranged horizontally in a housing. This places a limit on the size of the screen frame available for platemaking such that the screen frame should be accommodated within the housing. Thus, the necessary housing size has increased with the increasing screen frame size. This has caused an increase in the installation area, and hence reduced the space efficiency. Further, since the screen has been pulled by the movement of the thermal head in a pressing state relative to the fixed screen frame, expansion has easily been generated in the screen. This expansion has caused an error in the perforated platemaking pattern from the designed pattern, and hence caused the problem of degradation in the platemaking accuracy.